The present invention relates to an apparatus for compression molding articles made of plastics.
The invention can be applied non-exclusively to an apparatus for manufacturing any plastic articles, and in particular caps for closing containers and the like.
U.S. Pat. No. 5,807,592 by the same Assignee discloses apparatuses for pressure-molding of caps for closing containers, such as screw caps. Such apparatuses comprise a carousel that rotates about a vertical axis and on which a plurality of pressure-molding units are mounted concentrically around the rotation axis of the carousel and at identical angular distances. Each one of said units comprises an upper punch, which cooperates with a lower mold that is aligned with said punch and has a molding cavity.
By virtue of the rotation of the carousel, the molding units trace a circular path, which comprises a first sector, in which the necessary doses of plastic material to be molded are deposited in the cavities of the molds, a second sector, in which the article is molded, a third sector, in which the molded article is cooled, and a fourth sector, in which the molded article is extracted and conveyed away.
In these known apparatuses, the plastic material to be molded is removed from an extruder by means of a rotating head provided with a plurality of removal elements, which trace a circular path that has a point of tangency with the extrusion nozzle and with the circular path traced by the molding units. The rotating head and the carousel are mutually in step, in order to allow the removal elements to remove in succession doses of plastic material from the extruder and deposit them in the cavities of the molds.
Apparatuses for forming liners inside preformed caps are also known. In these apparatuses, the doses of plastic material, after being removed, are introduced in the preformed caps conveyed by a rotating head. The caps are then transferred into the molding units of a rotating carousel, which form the liners directly inside the caps.
The conventional apparatuses suffer the drawback that the doses of plastic material removed from the extruder, by moving in a free air environment, do not maintain a constant temperature until molding occurs and therefore have differences in the degree of plasticity of the plastic material between the surface layers and the innermost layers.
These differences cause variations in the flow of the plastic material during molding and generate defects on the finished product.
This drawback, moreover, is worsened by the removal elements, which cool down along the path that they trace, so that when they make contact with the plastic material at the outlet of the extruder they cause a localized temperature variation in the contact region, which causes aesthetic surface defects on the product.